Just to be clear, you have two smtp servers, and each account is sending on the server with the same user name when you open Tools/Account Settings, select an account in the left pane, then check the Outgoing Server (SMTP) drop-down box in the right pane.

Thanks for your response, to clarify I have: setup 2 mail accounts - both are google mail accounts. I have 2 outgoing SMTP servers both smtp.gmail.com. Each server has the correct username for authentication. I have given the servers unique descriptions, so that I am sure I have attached the correct server to the correct mail account. Everything is setup fine, they both receive mail, however I am only able to send email for one of the accounts, I simply get an unknown error.

I have a similar problem, getting "unknown command ..." on my second email account. This problem just started this past week; it has worked fine for the past 3-4 years.

I think I have discovered the problem: Google just alerted me to an attempt to sign in to my second account. I am required to enable "allow access to less secure apps" in order for Google to permit the sign in. So, Google is fine with my first email account (gmail) being accessed by Thunderbird, but a second (gmail) account requires allowing "less secure apps" access to be allowed. (??) It makes no sense (except that Google would like to push anything except Google apps out of the picture), but now I can access my second email account in Thunderbird.

Hey Ken, thanks for your response, however I have already allowed less secure apps within both my google accounts, so that isn't causing the problem. I have setup the same google accounts on thunderbird on a mac and windows 7 and they work fine. However on XP I am getting the unknown error. So it must be something to do with XP version of thunderbird that is causing the problem. Thanks again, J

J. Yes, must be related to XP. I failed to note that I am working under Win 10.